Changes to Master Slide not reflected in course

Hey, all. My company and I changed our slide format from standard to wide, but this stretched the background image we used on our Master slides. Because I do not have that original image, I just pasted new, proportionate logos on top of the image used in the Master slides. However, I when I close the Master view, I am not seeing those changes reflected on my slides, even when I reset the layout. Does anyone know why this might be happening? Thanks!